1. Field of the Invention
The present invention relates to a method for operating a braking system of a motor vehicle equipped with a wheel-slip control device, wherein the braking system includes an electrohydraulic service brake generating braking forces independently of an actuation by an operator, and a mechanical or electromechanical parking brake actuated by an actuating element.
2. Description of Related Art
It is generally known to provide specific actuating elements—such as handbrake levers or pedals, for example—for the parking brake in motor vehicles. Parking brakes have been constructed as purely mechanical parking brakes which, for example, are connected to the actuating element in the passenger compartment of the motor vehicle via a control cable. The operator or driver of the motor vehicle, actuates the actuating element—pulls the handbrake lever up or presses the pedal down—generating a torque on the handbrake lever or pedal transferred via the control cable into a tractive force that generates the locking force of the parking brake. In the case of electromechanical parking brakes, the force generated by the operator is replaced by an electromechanical force. Securing and releasing of parking brakes of such a type is instituted by an electric switch. Parking brakes are generally designed and suitable to hold the vehicle at a standstill by mechanical means, for example on an inclined roadway. The parking brake usually acts on the rear wheels of the vehicle.
With the aid of the service brake it is possible for the driver—with action that can be graduated—to decrease the speed of the vehicle during its operation or bring the vehicle to a standstill. Modern vehicles are equipped with an electrically controllable service-brake system capable of executing, in addition to an anti-lock braking function (ABS), an automatic braking function independent of operator actuation, for example a traction control system (TCS) or an electronic stability program (ESP). For this purpose, the service-brake system includes, in a known manner, an appropriately configured electrohydraulic service brake which, for example, is designed with an electrohydraulic control unit and/or with an electronically controllable brake booster, or a so-called “brake-by-wire” system. ABS, TCS, ESP and similar braking functions are realized by a device also designated herein as a wheel-slip control device.
Whereas a locking of the wheels of the vehicle upon actuation of the service brake is actively prevented, for example by the ABS function of the service-brake system, the parking brake is usually not suitable to lock the vehicle wheels braked by it, for example the rear wheels, during travel of the vehicle. Particularly on dry asphalt—a roadway with a high coefficient of friction—the parking-brake torque capable of being applied via the actuating element of the parking brake may not be sufficient to lock the corresponding wheels of the vehicle. This applies, in particular, to parking brakes that, for example, act on disk brakes of the motor vehicle capable of being actuated by the service brake. To bring about a locking of the wheels by the parking brake, a high expenditure of force is required, which usually is not capable of being applied by an average operator via the conventional actuating element of the parking brake.
Another known parking-brake concept enabling transmission of higher braking forces via the actuating element to the wheels, in particular rear wheels of the motor vehicle braked by the parking brake, provides a separate brake drum that can be acted on by the parking brake. The brake drum made available in a disk brake usually capable of being actuated by the service brake. However, a desired locking of the wheels braked by the parking brake cannot always be ensured under all operating conditions of the motor vehicle even with this braking concept. In addition, a parking-brake concept of such a type is associated with an increased manufacturing effort and gives rise to correspondingly higher costs.
In some situations, however, for example in dangerous situations and threatening situations or in motorsport, it is desirable for the driver of the vehicle to execute a so-called handbrake turning maneuver, or handbrake turn for short, in the course of which the vehicle is caused to adopt extreme drift angles—caused to oversteer—enabling turning the vehicle by 180° in an extremely short time, or to be able to take a particularly tight or—due to the roadway—difficult U-bends and S-bends. In this case, the rear wheels are caused to lock immediately by the parking brake retarding the wheels to allow the vehicle to skid. For example, in the motorsport field it is known to equip vehicles with special, particularly long parking-brake levers, wherein by virtue of the longer handbrake lever, a vehicle operator can apply the parking-brake torque required for locking the wheels. Purely hydraulic parking brakes are known as well from the motorsport field, but they are not permitted for road traffic, since these hydraulic parking brakes use the same hydraulic circuit as the service brake.
An electrohydraulic service brake is known from U.S. Patent Publication No. US 2014/0129107 A1 wherein the non-driven wheels of a vehicle axle can be secured against rolling away at a standstill, inasmuch as a hydraulic pressure is built up and maintained selectively for the service-brake circuits of these wheels, depending on an actuation of a switch by the driver. This capability can be utilized to bring about a spinning of the wheels of the driven axle of the vehicle, in order to increase, for example, the temperature of the spinning tires prior to subsequently driving away.
German Patent Publication No. DE 102010062816 A1 describes an electrohydraulic service brake, wherein an occurrence of a negative jump in the coefficient of friction of the roadway during a braking operation is monitored. Upon recognition of a braking maneuver on a roadway with a negative jump in the coefficient of friction the boost of a brake booster is set to a smaller value than in the case of a braking maneuver on a roadway without negative jump in the coefficient of friction.
U.S. Patent Publication No. US 2010/0114445 A1 discloses a driving-assistance system able to carry out an autonomous braking operation upon recognition of an obstacle in the path of travel of a vehicle. For the purpose of decreasing the braking reaction time, in the case of a hydraulic or electrohydraulic service brake, it is proposed to preload the brakes with the aid of an ABS pump of an ESP unit.
European Patent No. EP 2 342 110 B1 discloses an electromechanical service brake on a front axle of a vehicle, and of an electrohydraulic service brake on a rear axle of the vehicle. By means of an ESP unit, braking forces can be generated on the rear axle also independently of an actuation by an operator.
U.S. Pat. No. 6,254,202 B1 discloses an electrohydraulic service brake of a vehicle with an ABS control unit. In case of an emergency stop, additional brake pressure can be generated by a pump of the ABS unit in addition to the brake pressure normally generated by a master brake cylinder.
German Patent Publication No. DE 102007028070 A1 describes an electrohydraulic service brake for a motor vehicle with a regenerative brake unit and with an electric machine for driving/retarding the motor vehicle, wherein in the course of a regenerative braking operation the regenerative brake unit does not feed a volume of hydraulic fluid corresponding to the desire for braking into the wheel brakes but rather stores it in a hydraulic-fluid pressure accumulator of the regenerative brake unit.
Finally, the article “Electrohydraulic Brake System—The First Approach to Brake-by-Wire Technology” by Wolf-Dieter Jonner et al., SAE Technical Paper 960991, 1996, describes the general structure of so-called brake-by-wire braking systems.
Against this background, the present invention provides a method for operating a braking system having an electrohydraulic service brake, a mechanical or electromechanical parking brake, and a wheel-slip control device enabling a simplified implementation of a handbrake turn. The method requires no mechanical modifications in comparison with already known braking systems of such a type.
It should be pointed out that the features listed individually in the following description can be combined with one another in arbitrary, technically meaningful manner and demonstrate further configurations of the invention.